1. Field of the Invention
The present invention relates generally to an optical communication system and, more particularly, to an optical communication system in which an optical amplifier is disposed midway along a transmission channel or at one end thereof, and an optical communication method for use with the same.
The present invention also relates to an optical communication system which provides optical communication through an optical communication channel provided with at least one optical differential gain amplifier.
2. Description of the Related Art
In an optical communication system of the type which utilizes an optical fiber as an optical transmission channel or, optical spatial propagation, as an optical transmission channel, the attenuation of an optical signal over the transmission channel leads to a deterioration in the signal/noise ratio (S/N ratio) of the optical receiver. The result is deterioration in the quality of the received signal and impairment of the reliability of the optical transmission itself. For this reason, the attenuation of the optical signal presents significant problems.
To cope with the above problems, one method is considered. In this method, an optical fiber whose attenuation of optical signals is reduced is used as an optical transmission channel, and an optical amplifier is disposed midway along the optical fiber so as to amplify and retransmit the signal light.
FIG. 6 shows a conventional type of optical communication system provided with such an optical amplifier.
As shown in FIG. 6, the conventional optical communication system is arranged so that a transmitted digital signal 5 is input to a laser driving circuit 12. A semiconductor laser 13 is turned on or off in accordance with whether the value of the transmitted digital signal 5 is a "1" or a "0". A laser light signal, which is emitted when the semiconductor laser 13 is on, is transmitted to an optical amplifier 43 over an optical fiber 41. The optical amplifier 43 amplifies the light intensity which has been attenuated due to transmission over the optical fiber 41, and assures the required light intensity. The light output from the optical amplifier 43 is transmitted over an optical fiber 42 and is incident on an optical detector 21. The incident light is converted by the optical detector 21 into an electrical signal corresponding to the incident light intensity. The received light signal is amplified and delivered as a received signal 6 by an amplifier 22. In FIG. 6, there are also shown examples of transmission signal waveforms.
However, the optical amplifier 43 used in the aforesaid conventional optical communication system entails a number of problems. For example, the optical amplifier 43 not only effects optical amplification of signal light but also outputs high-level spontaneous emission due to direct current excitation. The result is a deterioration in the S/N ratio or the quenching ratio due to spontaneous emission and it has therefore been impossible to realize satisfactory transmission quality.
More specifically, the signal output from the optical amplifier 43 encounters the following problems: 1) The degree of optical modulation (on-off ratio) deteriorates with respect to the input light; and 2) So-called beat noise occurs due to interference between the input signal light and the high-level spontaneous emission due to direct current excitation, and this leads to a deterioration in the S/N ratio.